Frequency meter



Sep. 13, 1927.V

' v1,641,973 y, J. w. HoRToN FREQUENCY METER Filed April 1851922 l Jolie/ `14./ Hor/0n y 55 Aw.

Patented i Sept. 13, 1927.

y`U1\JITI.=.D STATES ,PATENT OFFICE, f

'JOSEPH W.-'HORTON, OE EAsT ORANGE, NEW JERSEY, AssrGNoRTo WESTERN ELEC- TRro COMPANY, INCORPORATED, OE NEW'YORK, N.Y., A CORPORATION OE NEW YORK.

FREQUENCY METER.

Application iledk April 18, 1092.2.: Serial-NO. 555,284. f.

7; This invention relates Vtoa methodfof and means fory measuring the frequency and hence wave length ofan alternating current, A Heretofore, it has been the .practice `to measure the frequency land thereby determine kthe wave lengtlrlpv-of an alternating current byy using some form of resonant transmission device associatediwith an apparatus to .indicate the amplitude of the transmitted wave. TO determine an unknown frequency or to adjust a source of current to a known frequency, either the resonance point of the resonant transmission device or the frequency -of the source, l5 as the case may be, is varied until the current flowing through the resonant device is a` maximum and hence the deflection recorded -in the indicating apparatus isa maximum. In order that maximum current may 20, be supplied to the indicating apparatus V the e resonant transmission device must be Y sharplyselective of the frequencyl of .the

current supplied to it. 'l f .f

The presenty invention is directed to an arrangement adaptedto utilize the attenua,- tion properties of a resonant transmitting 'i device to control theV deflection ,ofthe-indieating apparatus associated with it, whereby the frequency of the current supplied to the device or the adjustment of the resonance point of the device may be accurately def termined. To secure the desired indication the resonant transmitting device. included' in this arrangement is always tuned to a frequency different from that of the current transmitted through it. It will therefore be apparent that this apparatus does not use a circuit resonant to the frequency of the current being measured. 40 v As is well-known, if the frequency of thel current suppliedy tol a resonant'. transmitting` device, i. e., tuned circuit or filter, is

varied over an appreciable range, theimpedanco loss or current attenuation Y and 4 resonant transmitting deviceA varies Moreover, if the resonance point offa tuned circuit or filter, the characteristic curveofl hencey the transmission ,equivalent-of the;

mit with more or less attenuation, aredetermined by the characteristic curve. c

Thus if the resonance point of a filter is set at a definite frequency, say 1000 cycles, 1t is possible to determine definitely. from its characteristic curve the magnitude of its impedance for currents of other frequencies. For instance, its impedance for currentsv of v900, 940I and 980 cycles may be 150, and 25 ohms respectively. This property of a tuned circuit or filter is utilized for the `purposes'of this` invention. Since the change of deflec- I tion recorded by au indicator which measures ytransmission thru the resonant device is most marked atthel point of the given characteristic curve where the rate of change of current is a maximum, this point has been selected for the purpose of illustrating the principles involved in .this invention.

According to this invention,- a resonant vtransmitting device and a non-resonant net- Y work are alternately used to transmit current from. a. given source to Aan indicating circuit. The desiredk frequencyvdetermination of the source or tuning of theresonant circuit. is ascertained -by a. comparison of the attenuations exhibited by the two transmitting .circuits,.the resonant device being so adjusted that the frequency of the source occurs atl the steepest point of the kcurve representing the relation 4between `its attenuationand the applied frequency.

The primary object ofthe present invention is to accurately indicate the frequency ofthe current passing through a vresonant ,transmitting device. v

Another Objectis to cause the deflection recorded by the indicator to be dependent upon the attenuation of a resonant transmitting device for the current which is supplied y toit.

Stillv another object is to permit the indicationrecorded to Ibe accurately checked;

Ay furtherobject is to effect a frequency indication by a resonant .transmitting device l which is not sharply tuned tothe frequenc ofthe current` being measured.

An additional object is to compare they energy supplied to the input terminals of a selectivetransmitting device with that available at its output terminals.`

k A further Objectis toy set the frequency of carrier wave sources, or to adjust the selective transmission devices ordinarily associated with them, hy means o1 the ordinary apparatus included in the terminal equipment et a carrier wave transmission system.

For a complete understanding), of this in rention reference may be made to the tollowing' detailed description taken in connection with the attached drawing in which Fin'. l shows curves which illustrate current variations, and rate ot' current variations in a resonant transmittiner device with change ot :trequency et the wares supplied to the device. Fin'. i? shows a circuit arrangement tor pacticing the invention. Fig. 3 illus* trates the application of this invention to a line over which alternating,r currents of three dillerent frequencies are adapted to he transmitted.

In Fig. 1. the ordinates represent current values with reference to curve A and rate oit change of current with frequency when reiterred to curve B, and the ahseissie represent frequencies. Curve A. heine' an ordinary characteristic curve` serves to illustrate the amplitude ot the current transmitted through a resonant transmitting;r device when the lfreoniency o't the electric wave supplied to it is varied over an appreciable rance. or the amplitude of the current when the frequency o1 the source is maintained constant and the resonance point of the device is varied over a similar ange. As therein indicated the critical frequency is 1000 cycles. It now, the frequency ot the current. supplied hy the source is varied from 700 cycles upwardly and the resonance point of the resonant device is set at 1000 cycles, or it the resonance point ot the device is similarly varied and the requency oit the source is maintained at 1000 cycles, the value oit the current transmitted through the resonantdevice will increase to a maximum. as is indicated hy the point A on curve A. lllhen a current of 1000 cycles is supplied to the device. a current ot maximum amplitude will he transmitted therethrouirh. whereas, an increase or decrease ot the frequency, or fai-lation of the resonance point. will result in a decrease in the current amplitude.

The rate ot' chantre of amplitude ot the current transmitted through the selective circuit. is shown in curve B. Referring to this curro it will. he seen that the rate ot chance ot the currenthecomes a maximum at- 9-10 cycles. rapidly tails to Zero at 1000 cycles, airain increases to a second hut dilleront maximum at 1060 cycles and gradually tails oliVA to some lower value at 1400 cycles.

In order to make use of the attenuating properties ot a resonant transmitting device `l'or u'ave length or frequency measurement, or ior adjusting: the frequency of an alternal-ine' current source, an arrangement such as that shown in Fig. 2 is used. In this Figure, there is shown an alternatingcurrent source 1, which is adapted to he connected by a hey or switch to a pair ot' terminals ot a n A second pair ot terminals o1 the resonant device are connected by a key or switch f1 to a local circuit :includingr an indicating device. Y

The resonant transmission device 3 includes a variable condenser T which may be adjusted to tary the frequency selectivity o'f: the device in case such adjustment is dcsirablc. This device is also provided with couplingh resistances 3 adapted to render its selectivity independent ot changes in the reactances ot the circuits coupled thereto.

For low frequency systems the indicating instrument may comprise any sensitive voltage controlled instrument, hut tor high frequencies such as are used for carrier and radio transmission, a detecting device should he included in the indicating circuit.

The arrangement shown is adapted for use with high refniency systems and comprises a detector (l. which may he a three electrode space discharge device, associated with an indicating instrument A source 0 supplies, to the impedance control electrode of the device (l, a negative polari'fiing;r potential ot such value that normally there is suhstantially no current lloiv through the plate circuit of the detector.

The incoming high -li'reqlumcy waves transmitted through the device are supplied to the input circuit o't the detector. The posi.- tive halt cycle ot the detected waves serve to reduce the negative potential impressed upon the grid ot the device (l hy the source (la, whereby the amplitude ot the current llowing through the plate circuit and supplied to the indicator 5 is increased.

Referring to Fig'. 1 it will he seen that between 940 cycles and 1000 cycles respectively, the curve A is relatively llat-tomied` and hence for a limited range adjacent to lits resonant frequency the amplitude ot' the oscillations supplied to the detector (l will be substantially constant. Thus, lor a delinite frequency range between the points at which the rate of change ot eurent with change oit frequency is a maximum the indicator will record a substantially constant detlection. Aleffause ot this tact. an accurate determination ot the frequency or the proper adjustment ot' the transmittinirr device is attended with considerable dilliculty.

lilowever, the attenuation oil a resonant transmitting device for currents ot dillerent 'frequencies can he accurately determined from its characteristic curve, liy providinev a network the attenuation oit which is inde'- pendent ot retniency and is equal to that ot the transmitting device, at the point where the rate ot change of current is a n'iaximum, means is provided whereby the accuracy ol resonant lranslnittinnv device o.

lll)

the adjustment,` whetherof the frequency of Y the source or the selectivity of theitransmit- 'iOne pair of terminalsof thisnetwork are connected to contacts of the switch 2, while the `other pair of terminals ofthe network are connected tocontacts of thev switch 4L.

' `The network 8 is non-resonant and hence its attenuation is independent offrequency. This network is designed to have the same attenuation as the resonantcircuit 8 at the point onits characteristic curve where the rate of change of current amplitude or from another viewpoint, the rate of change of attenuation is a maximum.

Assume, as above, that ythe resonant circuit isdesigned for the transmission without yappreciable attenuation of waves of 1000 cycles and, as shown in curve B, the points at which the maximum rate of change of current attenuations occur are 940 and'1060 cycles. Further, let it be assumed that it is desiredy to adjust the generator to supply a current'of 940 cycles. i

The resonance point of the circuit 3 is setaccurately at 1000 cycles by 'adjusting the condenser 7 which is calibrated for this purpose and the deflection of the instrument 5 noted( `The keys 2 andy i are thcn'actuated to disconnect the lter 3 and substitute in its stead the network 8, the attenuation of which at the setting assumed, is the same' as that of the resonant circuit for 951.0 cycle current. If the deflection indicated by the i instrument is greater or less' thank that* recorded beforejthe substitution of the non` resonant network,'the frequency Aof vthe cur@ rent supplied by the source 1 is either above orfbelow 940 cycles The frequency of the current suppliedl by the source 1 should be adjusted until equal deiections are pro-rv ducedy when the filter 3 is included betweenY the source 1 and the indicator v5 andvwhen i. krthe filter is replaced bythe network 8. 'In

this manner the accuracy of the adjustment made may be checked.

AThe adjustment of a resonant transmit! ting device, to, render it selective for an alternating current of deiinite frequency, may be' accurately determined in a somewhat y v similar manner. ,F or example, vto adjust a lter to resonance at 1000 cycles. the procedure is as'follows A currentl known to be i of y9,40 cycles frequency is lalternately suppliedfto the/filter 8 andthe non-resonant circuit 8, thecondenser 7 being varied until equal deiiections 'arel .recorded inthe indi- The curve A lis not symmetrical about theA or 1000 cycle. axisand hence there are twofrequency points a and a', at whichjthe amplitude of the Vcurrent transmitted-is thek same, i. e. at 940 vcycles and 65.!in the vicinity of 1050l cyclesl Thepoints a? and a3 identify two other frequencies at which equalcurrent amplitude is transmiti ted, vthe former point indicating a frequency of M-cycles andthe latter about 930 cycles. Since maximum deflection of the indicating device is recorded at the resonance point of the ilter 8, namely, at 1000 cycles, a means of identifying these points is readily available. The points a and a" may be distinguished, since the point a (940 cycles) is Vobtained with an increasing de-v iiection while a (1050 cycles) is obtained with a decreasing deflection. Points a2 and ar may also be distinguished since a2' (1060 cycles) is obtained with a decreasing deiiection while a3 (930 cycles) is obtained, with an increasing deflection'. The points a and a?, as also a and a2 will be distinguished, because/at points a and a? respectively, the rate of change ofdetiection will bea maximum, whereas at a3' anda a lesser rate'of change of deflectionA will be indicated..y Again, the position of the resonance' point of the tuned circuit or ilterwill be roughly indicated bythe condenser setting which, in View of the fact that the calibration curve of the tuned circuit or'tilter Vis accurately known, will serve as a check in etfecting'the desired adjustment. The network 8 will serveas an additional 'means to `differentiate between the vpoints a and (1,3

on the one hand, andbetween al and a2' on `equal to that of the` resonant circuitlwhen the tuning of the latter is adjusted toqdifter` ent frequencies, A plurality f of movable: contacts 25 associated with the network 8 may be used for this purpose. Again these contacts 25 may be adjusted to vary kthe transmission equivalent of the lnon-resonant network over the Y possible` transmission range of a resonant circuit, in which case by using a resonant circuit the resonance point of `which may be set atdif'ferentfrequencies ora plurality of resonant'circuits having their resonance points fixed at different frequencies, the calibration curves of which are knownfor the differentfrequency settings, the apparat-us may be used for frequency calibration purposes. Y To accurately determine an unknown frequency any'well known type offwave meter, or a meter of the type described above including `varesonant device 3 and associated indicatorV 5 operating as yan ordinary wave meter, may be used to obtain an approximate frequency indication. A meter including a resonantv transmitting Ycircuit 3, characteristic curves of which for different frequency settings are known, which is designed to freely transmit a band of frequencies within which the unknown frequency lies, and a non-resonant network 48 having adjustable contacts is selected. The current of nnknown frequency is sup plied to the resonantl device and itsvariable condeiiser is adjusted to shift the resonant point of the device until the rate of change of deflection appears to he a niaxiinuni and the contacts are then adjusted until the inuiedanco of the network 8 matches the iinpedance of the resonant de vice at this setting'. The condenser 'i' and contacts 225 are thereafter adjusted until the exact point at which the inaxiniuin rate oi? change occurs is indicated. This will be identified by the indicator recording a deflection which does not change when the network S is substituted for the resonant device 3. The frequency of the current inay then be determined freni the cluiracteristic curve correspoinline` to the resonance setting of device l which is finally used. This is true for the reason that each clniracteristic curve of the resonant device Sil has a point (Iy and av point A such that the determination of one at once dctern'iinea-s the other. Consequently having' determined point (L we know the abscissa of point il which is the required unknown frequency. The adjustment of the nonsresonant circuit S therefore serves as a check` in that the known attenuation for which the network S is set should correspond to that of the point 1. or (t: on the characteristic curve A.

This arrangement may be applied to a circuit adapted to transmit a plurality of waves of different frequencies as, for in stance. in a carrier wave system. whether transmission eilected by nieans of physical circuits to `guide the waves or by free waves, as in radio transmission.

Fin'. 8 illustrates a. carrier current line 9 provided with three frequency channels having: band filters l0. l1 and l2 designed to freely pass currents of different frequency hands, for transmission over the separate channels. These filters are connected to the line ll at the terminals lil, 1il and 15, and to the channel circuits 10, 111 and 12 through the pairs of terminals 13"--12-51, ley-1lb and lrwl" respectively.

The networks 1G, 1T and 18 are siinilar to the element S. but are cach designed to siniulate the attenuation of a corresponding iilter at the carrier frequency. Thus, network 1G corresponifls to filter 10. network 17 to filter 11. and network 1S to filter 12. ln order to ad instthe carrier frequencies for the several channels. the filters are disconnected freni the line il and channel circuits lll, 1l and 1";5*1 and their terminals are in turn con I'lectcd lo the terminals 19 of the switch 20. rlhe channel circuits in like sequence are connected to the terminals nl. .in order to adjust the source associated with the llilter l() to the frequency which it is desired to transinit over the first channel, after disconnecting; all the filters frein the line and channel circuits described above, the terminals 13 and 13 will be connectedto the switch tern'iinals 197 the terminals 2l, will be con nected to the terniinals l'l" to which the source for that particular channel is connected and the key il() actuated to connect that source through filter lil to the detector (l and indicating;l apparatus o. rhe switch Q() will then be returned to normal and the key 22, associated wit-h the network lo. will be actuated to cause the current to flow through this network to detector (l to actuate the indicatingl apparatus o. The source is then adjusted until that frequency reached at which the deflection is .most rapid. lfj upon substitution of the network 1G, a dif ferent deflection produced the ope ator will proceed as described with reference to Fig. 2, to adjustthe local source until equal deflections are obtained in the indicator` Due to the fact that the frequency to which adjustment is to be niade is located. at a portion of the transmission characteristic of the iilter at which the rate of change of attenuation with frequency is very great, an extremely accurate setting lnay be effected. lly connecting` the filter terminals lll and 111a to the switch terminals 19, terminals la" to the terminals 21 and operating' the switch 23 associated with network 17, the frequency of the local source supplying' the second channel, :nay he investigated and adjusted if necessary. ln a similar manner the local. source for the third channel `may he tested. and its frequency adjusted if necessary. ln this ease the terminals l5 and 15L will be connected te 19, l5" to 21, and key 24, associated with the network 1S, will he operated. Any well known means5 such as a jump connection, may he used to connect the pairs of terminals 13-1l, lembi, or 14S-lotto the terminals 15) of the switch .2f 13", 1-l and 1.5b to the terminals 21. Conse-` quently, a detailed reference to this feature is believed to be unnecessary.

The word transmit and its derivatives are herein used in the broad sense, i. e. to deline transference of energy from one station to another, hence, so far as the wave nieter herein described is concerned, energy may be outgoing,` from or .incoming to the station in which the inctcr is used.

Moreover, while this apparatus has been described as applied to circuits which include resonant means having flat-topped characteristics, it not limited to use with resonant means of this type. On the contrary, it is to be noted that the operation of the nieter does not depend upon the adjustment of the resonant means to sharp selectivityVY for the. frequency of the wave supplied to it and hence it will be equally ellective whether used with resonant ineans' having1 sharp-topped or flat-topped eliar` acteristles.

0 and terminals kits output terminals. f

f l Obviously then, this meter is of Vapplication and mayy be used with low fre-I quency, carrier current, sometnnes referred l to as wired-wireless, for radio frequency systems', at the transmitting-l and receiving stations. and irrespective of vwhether the when it is used/,and adjusted in ,themanner set forth in this description,and hence the energy impressed vupon itsfinplit terminals bears a fixed relation to that impressed upon the indicatingcircuit.l ySince the attenuat-ion of the network is3 mademequal to 'y that of thexresonant `device for. the frequencyto ,whichadjustment yisto be effected, it Will be apparent `that thenetwork may properly be .saidy tofcompare the energy supplied to the y ,resonant device with; that available 1 at i Amplipfying means mayi-beused in'rthe ilocal circuit between thejdetector andjindicator., 5l if found desirable.

1 While in `the preceding description cerl tain specific details yandarrangements have been. set forth for thepurposeof` completely and clearly disclosing the nature of the invention and itsmethod of operatiomit 'is to ybeunderstood that this invention is not limitedy tol these specific features but only by the. scope of the appended claims.

What is claimed is:

l. A method ofmeasuring the frequency of an alternating current which comprises transmitting the current through a 'path having a predetermined frequency 'selection characteristic, and comparing the output energy from said path with energy having a fixed relation to the energy supplied toV said path.v

2. A method of measuring the frequency of an alternating current which comprises causing the alternating current to traverse a path the attenuation of which is a functionv of the frequency, causing said current totraverse a pathV the attenuation of which is independent of frequency but is definitely related to that of the first-mentioned path, and comparing the energies transmitted through said paths.

3. A method ofmeasuring the frequency of an alternating current 'which comprises causing the alternating current to traverse a path the attenuation of which is a function of the frequency, causing said current to traverse a path the attenuation of which is independent of frequency but is definitely related to that of the first-mentioned path at a point of predetermined selectivity, and comparing the energy transmitted through one path with the energy transmitted through the other path.

4. A method of measuring the frequency general'fof an alternating i .current which comprises causing the alternating current to traverse a path the attenuation of which is a functionl of the frequency, causing said current to lltra'versea path the attenuation of whichV` Yis independent of frequency but is related to that of the first-mentioned path 'at the point at whichv the rate of change of attenuationA is a maximum, and causing the energy ,transmitted through theV respective paths fto V produce equal indications.

5. A methodfof measuring the frequency 'lof an alternating current which comprises f causing the alternating current to traverse fa path the attenuation of which is a funcf tion of the frequency, causing said ,currentV ,to traverseV a path the attenuation of which is independent of frequency but is equall to that ofthe first-mentioned path at one point on its characteristic curve, and com- .y paring the energy transmitted through the said paths.

*6; A wavemeter comprisinga Vvvaveflter having a plurality yof sections.

7,'A wave ymeter comprising a resonant- `trangmission circuit and means for comparing the outputjenergy fromsaid filter ,with energyibearing a fixed relation to the energy: supplied. to said path.

8. A ,wave meter comprising a wave filter,Y

and means for comparing the output energy from said filter with energy bearing a fixed relation to the energyv input to said filter. f

9. A Vwave meter comprising a resonant circuit anda non-resonant circuit and means for interchangeably connecting said circuits between an alternating current source and an indicating apparatus. Y

10. A wave meter comprising a resonant circuit, a non-resonant circuit, means for adjusting the resonant point of said resonant circuit, means for adjusting said non-resonant circuit and means for interchangeably connecting said circuits between an alternating currentsource and an indicating apparatus.

l1. An electric apparatus comprising a circuit the attenuation of which is the function of the frequency, a circuit the attenuation of which is independent of frequency but is definitely related to that of the firstmentioned circuit at one point on its characteristic curve and means for interchangeably connecting said circuits between an alternating current source and an indicating appa- -means for interchangeably connecting said circuits between the alternating current source and an indicating apparatus.

land said related means between a source of y current supply and an indicating apparatus.

1i. An electric apparatus comprising a resonant circuit. ineans :tor adjusting; said circuit to predetermine a point of selected rate or' change ot attenuation, a non-resonant network the attenuation of which is equal to that oil said circuit at said point and means `tor interchangeably connecting said circuit and network between a source of current supply and an indicating;` apparatus,

15. An electric apparatus comprising a resonant circuit haring a selected rate of change of attenuation with change of 'lre` quency of the wares supplied thereto, a nonresonant network the attenuation ot which is equal to that et said circuit and means for interchangeably connecting` said circuit and network between a source otcurrent supply and an indicating; apparatus.

1li. nl Wave meter comprising; u circuit the attenuation of which is the function ot the frequency, a circuit the attenuation of which is independent oi 'frequency but definitely related to the point at which the maximum ate of change et attenuation oit the first- `Learners mentioned circuit occurs and means for intercliangeably connecting said circuitsbetween an alternating current source and an inilicating apparatus.

17. A ware meter comprising a resonant circuit, a source supplying?r waves of a delinite frequency thereto, an indicating apparatus, means i'or adjusting said circuit to predeterniine a point of maximum rate of change oi attenuation for the Wares supplied hy said source, a non-resonant network the ttenuation ot which is equal to that of said circuit at said predetermined point and means ilor interchangeably connecting said circuit and network between said source of supply and indicating apparat-us.

1S. An electric apparatus comprising a resonant circuit and means for indicatingr the adjustment et said circuit to predetermine a point et maximum rate ot change ot attenuation lor electric waves of definite frequency comprising;l a non-resonant network the attenuation ot' which is equal to that of said circuit at said predetern'iined point, means t( adjusting the network in accordance with the adjustinentI ot said resonant circuit, an indicator, and means for alternately connectine' the resonant circuit and the network to said indi 'atoiz y ln witness whereof, `I hereunto subscribe my naine this 17th day of April A. D., 1922.

JOSEPH W. HORTON. 

